1. Field of the Invention
The present invention relates to a method for assembling a fuel assembly in general, and relates in particular to an efficient method for fixating an upper nozzle to control rod guide pipes through sleeves.
2. Technical Background
In general, a fuel assembly such as the one illustrated in FIG. 6 is known. This assembly comprises: an upper nozzle 1; a lower nozzle 2; a plurality of grids 3 disposed between the nozzles 1 and 2 with predetermined spacing and each having grid spaces formed by intersecting straps; a plurality of guide pipes 4; and a plurality of fuel rods 5 disposed in corresponding cells of each grid. The guide pipes 4 are inserted and fixed to the grids 3 and to the nozzles 1, 2. The fuel rods 5 inserted into the grid cells are held elastically by the springs formed in the grids 3.
Conventional steps for assembling this type of assembly are as follows: first, a semi-assembled member 6 comprising the lower nozzle 2, a plurality of grids 3, the guide pipes 4 and the fuel rods 5 is prepared. Following this step, the upper nozzle 1 disposed above the semi-assembled member 6 is engaged with the top ends of the guide pipes 4, and the top ends are fixated to the upper nozzle 1 by welding to produce a fuel assembly.
In such a conventional process of assembling the upper nozzle 1 to the the top ends of the semi-assembled member 6, it is necessary to weld the upper nozzle 1 to the lengthy body of the semi-assembled member 6. Such a procedure demands expertise and is also time consuming. Furthermore, the welding operations are carried out in proximity to the fuel rods, and the workers were exposed to relatively high doses of radiation.
Therefore, there has been a need for an improvement in the assembling method, from the viewpoints of reducing the radiation exposure to the worker as well as achieving efficiency in assembling operation.
In the field of technology of fixing the upper nozzle 1 to the top ends of the semi-assembled member 6, other methods of assembling are known. For example, in one method known as the detachable upper nozzle method, the upper nozzle 1 is engaged with the top ends of the assembled member by means of inner sockets disposed on the top ends of the guide pipes; individual outer sockets formed within the upper nozzle 1; and locking member which keeps the inner sockets engaged with the outer sockets (refer to Japanese Patent, First Publication, Sho60-93988, JP, First Publication, Sho61-226686, JP, First Publication, Sho62-116290). Other methods include a bulging method, which involves engaging a sleeve disposed on the upper nozzle with the guide pipe and bulging the sleeve part (refer to the above-mentioned JP Sho62-116290, JP, First Publication, Sho55-125479, JP, First Publication, Hei2-196997).
Even in these methods, however, the fundamental problems still exist as in the conventional method of assembling the fuel assembly that there is a risk of radiation exposure to the assembly workers and that the assembling steps are time-consuming.